I typically use short rows on the back for two purposes:
1) To make the garment follow the slope of the shoulders.
2) To add length to the bottom so that it hangs parallel to the ground all the way around instead of dipping at the front.
For 1), the first short row just goes the width of the back of the neck opening, and then I add about 3/4" on each side at each turn until I get to final width of the garment. The right amount to add depends on how sloping or flat your shoulders are. Flatter shoulders = fewer turns with more stitches added per turn. Steeper shoulders = fewer stitches added at each of more turns.
For 2), I just eyeball it when I'm getting to the end and go back and forth across the entire back a time or 2 as needed.
HTH.
I learned all of the above from Barbara Walker's Knitting from the Top. Read her descriptions of short rowing if you get the chance.

Good response, Stan. I use the Walker method and had forgotten that I learned it from her until I read your comment. It amazes me as to how much Elizabeth Zimmermann and Barbara Walker revolutionized knitting. -- Books, knitting, cats, fountain pens...Life is Good.

Because of my goon build, it's a necessity for me to use short rows on the upper back. I will usually start 5 to to 10 sts from the ends, knit anywhere from 4-10 short rows (depending on my gauge-4 for bulky, 6-8 for worsted and 8-10 for DK/sport weight) and start probably around 2-4" down from the cast off. I hate to give you all the ranges, but even the row gauge in different weights will make a difference in how far down you should start. I typically have to start down further than most, because my upper back is pretty rounded, or convex, compared to my lower back which is concave. I will typically hold about a third of the front sts. in the middle to lower the neck front a couple of rows for a crewneck. I hope that helps! I've tried many of the short row methods, but I prefer the good old W&T and when I'm knitting my sts. with the wraps, I pull the wrap up and over the st. it wraps and work whichever decrease that ill put the stitch proper on the public side of the work with the wrap behind.